PROJECT SUMMARY/ABSTRACT Hypoplastic Left Heart Syndrome (HLHS) is the leading cause of neonatal cardiac death in the United States accounting for 2-3% of all congenital heart disease. While ?HLHS? is used to describe a spectrum of anomalies afflicting the left ventricle, it is defined as the severe underdevelopment of the left heart and ascending aorta along with atresia or stenosis of the mitral and aortic valves. Currently, the long-standing theory for the etiology of HLHS proposes that aortic or mitral valve obstructions impair blood flow through the left ventricle leading to hypoplasia of the left sided structures of the heart. While clinical trials have attempted to relieve blood flow obstructions in utero, these studies have had limited success in rescuing LV growth and blood flow in human HLHS fetuses. Here, I present preliminary findings that reveal that cardiomyocytes derived from HLHS patient- derived human induced pluripotent stem cells (hiPSC) demonstrate significantly reduced contractile force generation suggesting that intrinsic myocardial dysfunction may contribute to the disruption ventricular blood flow. These findings raise the exciting possibility of developing therapeutic strategies to potentially improve myocardial function to prevent LV hypoplasia in utero. The overall objective of this proposal is to identify molecular and transcriptional mechanisms giving rise to impaired contractile function in HLHS ventricular cardiomyocytes. The hypothesis is that HLHS ventricular cardiomyocytes display dysregulated expression of core transcriptional regulators necessary for the maintenance of the contractile machinery used for healthy myocardial function. Using the hiPSC model system, I will use healthy and HLHS patient derived hiPSCs to test the central hypothesis and attain the objective of this application. Specific Aim 1: Identify the molecular mechanisms giving rise to impaired contractile function in HLHS hiPSC-derived ventricular cardiomyocytes. Specific Aim 2: Determine the role of cardiomyocyte transcriptional regulators in cell function and survival in HLHS-hiPSC derived cardiomyocytes. Using CRISPR/Cas9 genome editing, I will introduce a genetic reporter system that will allow for the isolation of ventricular and left ventricular cardiomyocytes from hiPSC differentiation in vitro. This will allow for the study of specific sarcomeric and electrophysiologic mechanisms driving impaired contractile function and to assess whether these defects are specific to the LV. Using state of the art single cell RNA-sequencing technology, I will conduct extensive transcriptomic profiling of HLHS hiPSC-cardiomyocytes to identify dysregulated expression of key transcriptional regulators involved in regulating genes necessary for myocyte function and survival. Successful execution of the work proposed will lead to three significant contributions: 1) Will identify specific molecular drivers of myocardial contractile dysfunction in HLHS, 2) Will reveal, for the first time, whether myocardial contractile deficits and mechanisms are specific to the LV potentially explaining the left side specific lesions of HLHS 3) Will identify novel transcriptional pathways driving impaired contractile function in HLHS.